Conventionally, an air flow meter capable of detecting humidity is known, and Patent Document 1 (JP 5178388 B2 corresponding to US 2010/0031737 A1 and US 2012/0079879 A1) describes such an air flow meter. An air flow meter described in Patent Document 1 has a flow rate sensor and a humidity sensor. The flow rate sensor is disposed in an auxiliary air passage into which a part of intake air flows. The humidity sensor is disposed in a secondary auxiliary air passage branched from the auxiliary air passage. The secondary auxiliary air passage has an inlet through which air flows from the auxiliary air passage into the secondary auxiliary air passage and an outlet from which the air returns to the auxiliary air passage from the secondary auxiliary air passage.
However, according to studies conducted by the inventor of the present disclosure, dust may be adhered easily to the humidity sensor that is disposed in the secondary auxiliary air passage since large amount of air flows in the secondary auxiliary air passage. That is, the air flow meter may be contaminated easily by dust, in other words, the air flow meter may have a low contamination resistance.